In such units, one of the essential conditions to be respected in the absolute cleanness of the ambient air. This cleanness is evaluated by the detection of a very small number of particles with a size exceeding 0.5 microns in a unitary air volume and by the substantially total absence of corrosive gas in the ambient air. Such rooms, called "white rooms", have very high production and operating costs, so that the following factors should be limited;
(1) The ground surface or volume occupied by the machines contained in the white room.
(2) The air quantity force-extracted from the white room, because this involves the introduction into the white room of an equal quantity of air treated and filtered by absolute filters, which is very onerous. Such an air extraction is mainly brought about by exhauster hoods over the machines producing the polluting emanations.
(3) The air quantity escaping from the white room during the opening thereof, particularly during the introduction into it of large parts, because the white room is under an overpressure.
(4) The white room opening frequency, because this always leads to a drop in the overpressure and causes movements which introduce dust.
(5) The liquid or corrosive gas quantity able to escape into the air of the white room.
Furthermore certain machines, such as deposition furnaces, are present in the white room. A deposition furnace essentially comprises a quartz tube serving as a reaction chamber. Each deposition furnace must be periodically extracted and treated to remove the deposits from its surface. This cleaning operation (or, more precisely, the operation of restoring the initial cleanness conditions) consists of immersing the tube in a stirred bath of extremely corrosive chemical products, generally a hot, concentrated mixture of HF and HNO.sub.3. In this way, it is possible to remove tungsten deposits, which are particularly resistant.
As quartz tubes are very fragile and long (i.e., approximately two to three meters, the cleaning operation is difficult. Treatment of the tube in the white room takes up space, requires a large air extraction for limiting the contamination of the air by the corrosive emanations and does not completely eliminate the risk of damaging the quartz tubes.
A treatment outside the white room involves handling the tube for removing it and returning it to the room. This entails the risk of dust or polluting materials being deposited on the tube during its stay outside the white room. Moreover, in this case the handling of the tube for removing it and returning it to the white room requiring a significant opening of the room. This entails a high consumption of filtered air and a pressure drop within the white room.